It is common practice in air sampling pumps to maintain the air flow at a constant rate independent of buildup in back pressure by direct measurement of the air flow using a suitable sensor, the signal from which is then used to control the speed of the vacuum pump such that constant flow is maintained.
In theory, the volume or rate of air delivered by a small vacuum pump such as is commonly used in a personal air sampler may be controlled and/or measured by employing the known characteristics of the pump in a microprocessor or the like, and comparing the known characteristics to current inputs representing flow, perhaps modifying the inputs by other variables such as outside air pressure or temperature. Various workers in the art have used such approaches to the problem of maintaining accurate air flow readings for use in calculating the concentrations of air contaminants. See, for example, Peck et al, U.S. Pat. No. 5,107,713. These patentees establish a table of values which relate the pump motor's RPM to air flow rates, store them in a microprocessor, and modulate the current pulse width as a function of deviations from the known RPM/flow relationship. Baker et al, in U.S. Pat. No. 4,527,953, vary the current to the pump, and hence its output, also by varying the current pulse width, but as a function of the relative durations of open and closed positions of a differential pressure switch located parallel to an adjustable orifice in the air conduit. Baker et al try to keep the air flow smooth with the aid of an "accumulator" which may be milled or molded into the frame of the pump and covered on one wall with an elastomer sheet (col. 4, lines 17-24). Thus Baker's accumulator is positioned upstream of and removed from his pressure sensor. In an earlier U.S. Pat. No. 4,269,059, Baker places his accumulator between the filter on the intake and the pump.
Pulse width modulation is also used by Hampton et al in U.S. Pat. No. 5,269,659 to control the air pump, this time as a function of pressure differential across a Venturi. Betsill et al, in U.S. Pat. No. 5,163,818, use a programmable computer to calculate flow from a number of variables, and regulate the voltage to the pump motor to maintain a desired air flow rate. In U.S. Pat. No. 5,520,517, Sipin bases the pump motor control on changes in load, sensed by changes in pressure and speed and compared to pump characteristics stored in memory.
Systems relying on known characteristics of pumps assume to one degree or another that the pumps and motors controlling them will not change, but it is known that pumps and motors will wear, lubrication will change with age, gaskets and bearing surfaces will erode or otherwise deteriorate, and various other problems arise to change the response of the pump to a current of a given characteristic.
Also, as observed by Lalin in U.S. Pat. No. 4,532,814, issued Aug. 6, 1985, "(a)ll known pump sampling systems which control flow by adjustment of pump motor speed produce an air flow with relatively high pulse undulations particularly at low flow levels. With a highly pulsed flow it is difficult to set the flow rate." Column 1, lines 59-63. Lalin maintains a steady flow of gas or air to a sample collecting device by providing a supplementary flow of gas or air to the inlet of the pump in response to a signal representing pressure differential across an orifice in the air or gas conduit, thus avoiding controls on the pump motor. Settings for the flow control valve are manually adjusted. The supplementary flow to the inlet of the pump may be taken from the outlet of the pump. See also the continuation-in-part application and U.S. Pat. No. 4,576,054.
Lalin also, in U.S. Pat. No. 5,562,002, disclosed a device for damping the pump pulsations in a reciprocating piston flowtube comprising a diaphragm and a porous member having open channels. The piston drops by gravity to the bottom of the flowtube; the diaphragm is supported by the porous member.
In U.S. Pat. No. 5,000,052, Sipin shows a laminar flow sensing element which comprises a stack of individual flow channels (col. 13, lines 58-68).
None of the above constructions provide a simple device for measuring air or gas sample flow which neutralizes the effect of the back pressure caused by the strokes. The approach of Bossart et al, in U.S. Pat. No. 5,295,790, requires a special laminar flow element such as porous member 21 in a suitable housing, which will "simulate this linear relationship between the flow rate and pressure drop in a portable personal sampling pump."(col. 3, lines 49-51), provided the air flow is maintained at a low Reynolds number. While Bossart et al purport to be able to simulate linear relationship, they do so at the cost of providing the special laminar flow element. The porous element preferred by Bossart and the small orifice used by Baker and others are susceptible to the buildup of particles from the air which can clog up the orifices through which the air must pass, and across which pressure drop is measured. It should be noted that a major use of air sampling pumps of this type is for dust measurement and even with a suitable in-line filter, small particles can still pass into the sensor causing the buildup of changes in calibration over time.
The reader may also be interested in Simon et al U.S. Pat. No. 5,621,180, which uses a coiled capillary tube of 5 cm to 5000 cm length and in internal diameter up to 0.53 mm to control the flow of gas for a predetermined time into an evacuated sample vessel.